Antonio Machado (translated by Franciso Valera)

Nobel Laureates in Physics Albert Einstein (left) and Niels Bohr (right) walking.
Photo taken at the 1930 Solvay Conference in Brussels.
Source: Danish Film Institute
Photo: Paul Ehrenfest

“Let a railway wagon be moving along a track at a constant speed . Let a man walk along the wagon at a speed  in the direction of the wagon’s motion. By which speed  relatively to the railway embankment is the man moving during his walk? It seems that is only one possible answer results from this way of thinking:
If the man stopped after one second, he would, relative to the embankment, have moved forward for a certain distance  which is equal to the speed of the wagon. Actually, relative to the wagon, that is, in relation to the embankment, he would also have traveled forward by a pace the distance , which corresponds to the speed of his walk. Thus, relative to the embankment, in the given second the man travels in all the distance

(19.1)

Later on, we will see that this way of thinking which is in accordance with classical mechanics, expresses the addition theorem, cannot be retained, and that this law, we had just now written, does not represent the truth”